Analytical instruments are used extensively in production as well as in research settings. The quality of the process or research depends in large part on the accuracy and precision of the analytical instrument. Unfortunately many of these instruments are prone to drifting and other forms of systematic biases. To counteract this, analytical instruments are periodically recalibrated using a known standard. The biases present in a particular instrument are not always linear, however. Thus, simply recalibrating the instrument more often will not always result in a more accurate analysis. Furthermore, the cost of the analysis in time and money increases with increasing rates of calibration, and so it is not always cost effective to increase the frequency of calibrations even when slightly more accurate results could be obtained. Accordingly it is desirable to be able to identify and characterize the systematic biases in a particular instrument so that an optimum calibration frequency can be determined.
One method of determining the optimum recalibration period is described by Walter Ligget, in "Tests of the Recalibration Period of a Drifting Instrument", National Bureau of Standards, Gaithersburg, Md. 20899. The method described by Ligget assumes a linear model to characterize the dependence of drift with time. Instruments whose responses oscillate do not correlate closely with a linear model, making this method inappropriate for some applications. Furthermore, the method described by Ligget only uses the upper half of a Fourier Transform to determine whether the instrument response is random by determining spectral flatness. The lower half of the transform is not used as the method requires that the spectrum be tapered to eliminate biases due to low frequency variations. The method also requires that outliers and linear trends be identified and removed before the spectral flatness is determined. These requirements reduce the versatility of the method and eliminate possibly relevant data from consideration.
Another method was described by H. C. Smit in the Journal of Research of the National Bureau of Standards, 90, 441-450, (1985). This method, like the method described by Ligget, assumes a linear model to describe the dependence of drift with time. Therefore this method also suffers from some of the same disadvantages as the method described by Ligget. Specifically, this method is not satisfactory for use in instruments where the response oscillates over time.